1. Field of the Invention
The present invention relates to a triangular-wave generating circuit, and more particularly to a triangular-wave generating circuit that can be synchronized with an external circuit.
2. Description of the Related Art
A pulse width modulation method is widely used in a power supply apparatus such as an inverter that converts a direct current voltage into an alternating current voltage or a DC/DC converter that converts a direct current voltage into a direct current voltage, a motor driver circuit that drives a motor, or the like. In order to perform pulse width modulation, an error between a voltage to be controlled and a target voltage is amplified by an error amplifier, and an error voltage thus obtained is compared with a periodic voltage having a triangular-wave shape and having a constant frequency with use of a comparator, so as to perform the pulse width modulation.
For generating the periodic voltage having a triangular-wave shape used for such purposes, an analog triangular-wave generating circuit disclosed in the patent document 1 or the patent document 2 is widely used. The triangular-wave generating circuit charges and discharges a capacitor, and performs charging and discharging alternately by comparison with two threshold voltages, so as to generate a desired triangular-wave signal.
[Patent Document 1] Japanese Patent Application (Laid-Open) No. 2004-72657
[Patent Document 2] Japanese Patent Application (Laid-Open) No. 2001-345682
[Patent Document 3] Japanese Patent Application (Laid-Open) No. 2004-242403
Here, there may be cases in which it is desired to generate triangular-wave signals that are synchronized with each other in two different semiconductor integrated circuits (hereinafter also referred to as IC). For example, a case is considered in which two inverters are disposed at both ends of a fluorescent lamp that is used as a back light unit of a liquid crystal television set, and driving voltages having opposite phases are applied.
In the event that the phases of the triangular-wave signals used in performing the DC/AC conversion are shifted from each other in the two inverters, the voltages applied to both ends of the fluorescent lamp will not have opposite phases, thereby generating an inconvenience in the light-emitting state. Other than this, there may be a case in which it is desired to generate a triangular-wave signal that is synchronized with an external signal.